Canvas sails had been used for thousands of years when German engineer Anton Flettner realized that rotating cylinders might work as well: The spin produces a difference in pressure on opposite sides of the cylinder, and this can propel a ship through the water.

The idea made a splash when it was introduced in the 1920s, and an experimental vessel crossed the Atlantic in 1926, but the rotating drums consumed a discouraging amount of power and Flettner moved on to other projects. The principle has been revived lately, though, in hopes of increasing the fuel efficiency of conventionally powered ships.